This invention relates to a coating method and more particularly to the improvement of a method of applying a coating agent to one or both surfaces of a thermoplastic sheet (including film).
A stretched thermoplastic sheet is widely used in industrial and miscellaneous fields due to its superior mechanical and chemical properties as well as a high dimensional stability. In many cases the sheet is put to practical use after subjected to a surface treatment such as printing or coating (hereinafter non-surface-treated sheet is referred to as "base sheet"). One problem associated with the application of such surface treatment to obtain a final product is that the adhesion between the base sheet and final surface treating agent is not always satisfactory. Therefore, it is generally considered effective to apply a surface treatment to the base sheet before application of such a final surface treatment. In many cases, moreover, the base sheet is generally lacking in slipperiness, so in uses where a very high transparency is strongly required of the base sheet, the base sheet is preferably coated on its surface with a slipping agent with a view to improving the processability in subsequent treatments.
To coat a biaxially stretched thermoplastic film with a liquid coating agent for such purpose, the use of the following means has heretofore been generally known: metering bar, direct gravure, offset gravure, 2-roll pressure, 3-roll pressure, 2-roll reverse, 3-roll reverse, dip coating, 1-roll kiss coating, 2-roll kiss coating, trailing blade, nip coating, inverted knife, air knife, floating knife, blanket knife.
The coating methods using these means usually are applied to films 100.mu. or smaller in thickness. But it is the present situation that in case such methods are applied to sheets having a thickness not smaller than 150.mu., disadvantages such as the occurence of defect of coating layer, lowering of working efficiency and irregularity in application are unavoidable. Among such coating methods, the one using a metering bar is regarded as being superior in point of uniformity of coating and working efficiency. But in case the conventional method wherein a metering bar is driven in reverse at low speed using a bar holder is applied to the coating for a thick sheet, there often occurs wire breaking and not only it is impossible to effect an industrial-scale operation stably for a long time, but also scratch trouble to the sheet and the occurrence of defects of coating layer caused by air bubbles are unavoidable. In an attempt to remedy such disadvantages associated with the metering bar system there has been proposed a method in which a liquid is fed to the metering bar. This method may be effective for reducing defects of coating layer, but it is difficult to prevent the occurrence of wire breaking during operation and scratch on the sheet.
It is an object of this invention to solve the problems associated with the prior art methods.
It is another object of this invention to provide a method capable of coating a thick sheet with a liquid coating agent uniformly over a long period of time without defects of coating layer, scratch on sheet or wire breaking.
It is a further object of this invention to provide a method of coating a sheet in the course of a sheet manufacturing process involving a stretching step.
Other objects of this invention will be apparent from the following description.